In general, the present invention relates to a method of dry, plasmaless etch of MEMS (micro-electro-mechanical systems) structures, which includes a release step in which gaps are opened (etched) between various structural surfaces. For example, surface machined cantilever beams and lever arms may be fabricated by etching away sacrificial layers within a structure; or, thin membranes or diaphragms for sensors or pumps may be created by hollowing out areas of a structure.
Micromachining technology compatible with semiconductor processes is used to produce a number of devices such as piezoelectric motors containing cantilever beams, hinges, accelerometers, reflector antennae, microsensors, microactuators, and micromirrors, for example. One of the most popular microactuators is an electrostatic comb driver, due to its simplicity in fabrication and low power consumption. Surface micromachining fabrication processes for the electrostatic comb driver, as well as other beams and lever arms, have problems with stiction of such beams and lever arms to an underlying layer over which the beam or arm extends. The lever arm becomes deformed from its intended position, so that it does not extend out as desired. In the case of a membrane or diaphragm, the membrane or diaphragm becomes deformed from its intended position and may become stuck to an adjacent surface. Stiction is the number one yield limiting problem in the production of the kinds of devices described above.
FIGS. 1A through 1C are simple schematics showing a cross-sectional side view of a starting structure for surface machining of a lever arm, the desired machined lever arm, and a lever arm which has been rendered non-functional due to stiction, respectively.
The FIG. 1A structure shows a substrate layer 102 (typically single crystal silicon), a portion of which is covered with a sacrificial layer 104 (typically silicon oxide), and a lever arm layer 106 (typically polysilicon) which is in contact with and adhered to substrate layer 102 at one end of lever arm layer 106. FIG. 1B shows the FIG. 1A structure after the removal of sacrificial layer 104 to produce the desired free-moving lever arm 107. The height xe2x80x9chxe2x80x9d of gap 108 between lever arm 107 and substrate 102, the length xe2x80x9clxe2x80x9d, and the cross-sectional thickness xe2x80x9ctxe2x80x9d of the lever arm 107 depend on the particular device in which the structure is employed. In many instances the relative nominal values of xe2x80x9chxe2x80x9d, xe2x80x9clxe2x80x9d, and xe2x80x9ctxe2x80x9d are such that capillary action during the fabrication process; or contaminants formed as byproducts of the fabrication process; or van der Waals forces; or electrostatic charges on the upper surface 110 of substrate layer 102 and/or on the undersurface 112 of lever arm layer 106 may cause lever arm 106 to become stuck to the upper surface 110 of substrate layer 102. This problem is referred to as xe2x80x9cstictionxe2x80x9d, and is illustrated in FIG. 1C. Stiction may occur during formation of the lever arm 107 or may occur subsequent to fabrication of the device and during packaging, shipment, or use (in-use stiction) of the device. A single crystal silicon or polysilicon surface of the kind which is frequently used to fabricate a lever arm, beam, membrane or diaphragm is hydrophilic in nature, attracting moisture which may cause stiction.
Various processes have been developed in an attempt to prevent stiction from occurring during fabrication of micromachined arms and beams. For example, in U.S. Pat. No. 6,027,571 to Kikuyama et al., issued Feb. 22, 2000, the inventors describe a wet etching process for micromachining, where the wet etchant preferably includes a surfactant. (Abstract) The surfactant is said to improve wetability during etching so that etching uniformity of a silicon oxide film is improved; in addition, if a silicon surface is exposed during the etching, the roughness of the surface can be suppressed by the surfactant (Col. 2, lines 58-64). Crystalline particles which are byproducts produced during etching can be prevented from adhering to the wafer surface by adding surfactant to the surface treatment. (Col. 3, lines 29-32). The surfactant is evidently used in an attempt to reduce some of the factors which contribute to stiction.
In U.S. Pat. No. 6,069,149 to Hetrick et al, issued Aug. 1, 2000, the inventors disclose a method for fabricating an adhesion-resistant microelectromechanical device. Amorphous hydrogenated carbon is used as a coating or structural material to prevent adhesive failures during the formation and operation of a microelectromechanical device. (Abstract) The amorphous hydrogenated carbon (AHC) coating is applied on the micromachined device after removal of the sacrificial layer and release of the structure. The sacrificial layer is removed in a wet etching solution such as hydrofluoric acid or buffered HF acid. (Col. 7, lines 26-32.) The method is said to reduce adhesive forces between microstructure surfaces by altering their surface properties. The AHC is said to create a hydrophobic surface, which results in lower capillary forces and an associated reduction in stiction. (Col. 2, lines 66-67, continuing at Col. 3, lines 1-4.)
An article in IEEE Electron Devices Magazine, IEDM 96-761 (1996 IEEE), entitled xe2x80x9cFabrication of Surface Micromachined Polysilicon Actuators Using Dry Release Process of HF Gas-Phase Etchingxe2x80x9d by Jong Hyun Lee et al. describes a process developed for the dry-release of sacrificial oxide in polysilicon surface micro-machining. Using anhydrous HF gas with a CH3OH vapor catalyst (Page 30.1.1, last paragraph, first column), the authors successfully fabricated a vibrating micro-gyroscope structure with xe2x80x9cvirtually no process-induced stictionxe2x80x9d. (Abstract). The authors describe how one of the major issues in surface micromachining is process-induced stiction of highly compliant microstructures to an underlying layer. The stiction of the microstructures is attributed mainly to capillary forces developed during a drying step which follows wet etching of the sacrificial layer. The Lee et al. article references reports by other researchers which describe other methods which have been used in an attempt to solve the stiction problem. For example, Lee et al. mention the use of a micromechanical temporary support, sublimation of the final liquid (present after etching of the sacrificial layer) by freeze-dry, temporary photoresist support with subsequent plasma ashing, removing the final liquid through supercritical state, or using low surface tension liquids. The supercritical dry method is said to generate excellent results, but this method requires the use of high pressure equipment which is not desirable in a fabrication process, due to equipment cost and safety issues.
Applicants"" review of the background art in general has indicated that stiction is the primary cause of low yield in the fabrication of MEMS devices. As mentioned above, stiction is believed to result from a number of sources, some of the most significant being capillary forces, surface contaminants, van der Waals forces, and electrostatic attraction. Factors which may contribute to stiction include: warpage due to residual stresses induced from materials; liquid-to-solid surface tension which induces collapse; drying conditions during processing; adverse and harsh forces from wet baths; aggressive designs (i.e. long and thin beams); surface-to surface attractions; inadequate cleaning techniques; aggressive cleaning techniques; and environments subsequent to fabrication, including packaging, handling, transportation, and device operation.
To reduce the probability of stiction subsequent to release of a beam, lever arm, membrane, or diaphragm (so that it extends over open space), a surface treatment may be applied and/or a coating may be applied over freestanding and adjacent surfaces. However, in some instances stiction occurs during the release process and prior to application of a stiction-preventing coating.
The present invention relates to a method of fabricating a MEM structure which reduces stiction due to fabrication process induced variables and which contributes to a reduced probability of stiction occurring during handling, transportation and device operation after fabrication.
The present invention pertains to a method of fabricating a surface within a MEM which is free moving in response to a stimulus. The free moving surface is fabricated in a series of steps which includes a release method, where release is accomplished by a plasmaless etching of a sacrificial layer material, followed by a cleaning step in which by-products from the etch process and other contaminants which may lead to stiction are removed. There are a series of etch and then clean steps so that a number of xe2x80x9ccyclesxe2x80x9d of these steps are performed. The release method may be followed by a surface passivation step in which cleaned surfaces are passivated and/or coated, so that they repel elements and compounds which may be present in the ambient atmosphere which may induce stiction. The coating may also reduce charge accumulation on feature surfaces, so that the possibility of stiction by electrostatic attraction is reduced.
In the cyclic etch/cleaning procedure, a portion of a sacrificial layer is removed, followed by a cleaning step, and the process is repeated until the desired amount of sacrificial layer is removed. In some instances, the surface properties of the structure surfaces which were in contact with the sacrificial layer, and which remain after removal of the sacrificial layer, are not as desired after the final cycle. These surfaces are then treated to provide the desired surface properties. The number of etch/clean cycles required in a given instance depends on the dimensions of the free moving structure which is being fabricated.
When the sacrificial layer is an oxide, the etchant used to remove the sacrificial layer is typically a fluorine-containing etchant. When the sacrificial layer is an organic polymeric layer, the etchant used to remove the sacrificial layer is typically an oxygen-species containing etchant. When the sacrificial layer is a metal-containing layer, the etchant is typically a chlorine-containing etchant. The etchant is selected to etch the sacrificial layer more rapidly than other layers exposed to the etchant, and to minimize or avoid the formation of chemical compounds which are harmful to the MEM surfaces which remain after removal of the sacrificial layer. The cleaning agent used depends on the byproducts produced during etching of the sacrificial layer, the ease of removal of the cleaning agent (along with the byproducts which are removed with the cleaning agent), and the surface properties which are generated on the structure surfaces which are contacted by the cleaning agent.
In a first embodiment of the invention, the sacrificial layer is an oxide, and the structural surfaces adjacent the oxide include at least one of single crystal silicon (silicon), polysilicon, or silicon nitride. The etchant for removal of the oxide is a vapor of an HF/catalyst mixture. The most advantageous catalyst is water, as water provides a faster etch rate; however other polar molecules which can provide OHxe2x88x92 ions may be used as a catalyst. Examples of other catalysts include chemical compounds which can be present in the vapor state under the same process conditions at which HF is in a vapor state, such as chemical compounds having the formula CxHy(OH)z, where x ranges from 1-3, y ranges from 3-9 and z ranges from 1-2. Alcohols and ketones work well. Chemical compounds having the formula CaHbOc, where a ranges from 1-3, b ranges from 2-8, and c ranges from 2-4 may also be used, such as acetic acid. Typically the catalyst concentration in the HF/catalyst mixture is less than about 25% by volume. It is important that the HF/catalyst mixture be maintained as a vapor in the process chamber, with the exception of a thin film (a few monolayers) on the surface of the substrate. Thus, the catalyst concentration in the HF/catalyst vapor is dependent on the temperature and pressure under which the etching of the sacrificial oxide layer is carried out. FIG. 2 provides a phase diagram for Anhydrous Hydrogen Fluoride which is generally indicative of the temperature and pressure conditions under which a vapor of HF can be maintained. This phase diagram has to be adapted for presence of the catalyst.
In the case of an HF/water mixture, where the ratio of HF:water is 10:1 or greater, the substrate temperature during etching is maintained between about 25xc2x0 C. and about 50xc2x0 C., and typically is maintained below about 45xc2x0 C. The temperature of the process chamber walls is generally slightly higher than the substrate temperature, to prevent condensation. The pressure in the process chamber is slightly below that which would provide general condensation of the HF/water mixture on the substrate; fine tuned to provide the thin film monolayer of condensed HF/water mixture on the substrate surface. For process integration reasons, it is helpful when the process chamber is operated at less than one atmosphere of pressure, and the substrate temperature may be adjusted to accommodate operation at such pressure. Desirable operating pressures range between about 300 Torr and about 600 Torr, for example.
The cleaning agent used subsequent to the HF/catalyst etchant mixture is a vaporous chemical compound which is polar in nature. Examples include chemical compounds having the formula CxHy(OH)z, where x ranges from 1-3, y ranges from 3-9 and z ranges from 1-2. Methanol, ethanol, and isopropyl alcohol (IPA) have been demonstrated to perform well as cleaning agents. Ketones such as acetone are expected to work well also. Additional example cleaning agents include chemical compounds having the formula CaHbOc, where a ranges from 1-3, b ranges from 2-8, and c ranges from 2-4. Acetic acid, which is a compound having this formula, performs well as a cleaning agent.
In a second embodiment of the invention, the sacrificial layer may be an organic polymeric layer, and the structural surfaces adjacent the organic polymeric layer may include a metal. The etchant for removal of the organic polymeric sacrificial layer in this instance is typically an oxygen-containing active species which oxidizes the polymeric layer into a volatile reaction product which is easily removed from a process chamber. The cleaning agent used to remove reaction byproducts and contaminants may be one of the cleaning agents described above with reference to the first embodiment.
In a third embodiment of the invention, the sacrificial layer may be a metal-containing layer, and the structural surfaces adjacent the metal layer may include an oxide. The etchant for removal of the metal sacrificial layer in this instance typically is a chlorine-containing active species which reacts with the metal containing layer to provide volatile reaction products which are easily removed from a process chamber. The cleaning agent used to remove reaction byproducts and contaminants may be one of the cleaning agents described above with reference to the first embodiment.
An important feature in all embodiments of the invention is the use of more than one etch/clean cycle to fabricate the free moving structure such as a lever arm, beam, membrane or diaphragm, for example. The number of etch/clean cycles required depends on the feature being etched. For a beam or a lever arm, the cross-sectional dimensions of the arm, the unsupported length of the arm and the gap between the arm and the underlying substrate are important factors. For a beam or arm having an effective cross-sectional radius in the range of 2xcexc or less, the longer the unsupported length of the arm, and the more narrow the gap between the arm and adjacent substrates, the more easily the unsupported arm or beam length can be deformed, and the larger the number of cycles which are necessary to avoid stiction during the fabrication process. The aspect ratio of the gap can be used to estimate the required number of cycles. The aspect ratio of the gap is the ratio of the length of the gap (the unsupported length of the beam or lever arm) to the minimum cross-sectional dimension of the gap. As a starting point, the aspect ratio can be maintained at about 1:1 and the number of cycles used can be nominally in the magnitude of the aspect ratio. For example, if the aspect ratio is 20:1, about 15-30 cycles may be used. One skilled in the art can adjust the number of etch/clean cycles depending on the results obtained from this starting point.
When the structural surfaces remaining after removal of the sacrificial layer include polysilicon or silicon nitride, surface treatment and/or application of a coating material may be helpful in reducing the tendency for stiction subsequent to device fabrication. Examples of coating materials which may be applied after removal of the sacrificial layer include alkylsilane-containing molecules, particularly an alkylhalosilane which is applied after oxidation of exposed substrate surfaces. Other surface coating materials such as primary alkenes may be applied over (reacted with) hydrogen terminated silicon (hydrogen terminated silicon may be achieved by treating a silicon surface with ammonium fluoride). However, the aklyslilane-containing molecules provide the advantage that they may be applied under similar process chamber conditions as those used in the etch/clean fabrication process. This makes it possible to carry out a release process in which a free moving structure is created and then to provide surface passivation and/or coating of fabricated substrates in the same process chamber, when necessary to prevent stiction. In the alternative, the etch/clean process may be carried out in one chamber of an integrated system, with the released structure then moved under controlled conditions into an adjacent process chamber where surface passivation and/or coating is carried out.